1. Field of Invention
This invention generally concerns isolated converter circuitry and more particularly relates to means for providing an initial bias and an enable signal for the control circuit referenced to the output of converter.
2. Background Discussion
It is a common problem in isolated converters to provide a proper bias for both primary and output circuitry, particularly during start-up or restart of the converter. Usually a controller (pulse width modulated (PWM) is one example) is on the input side and the feedback signal is provided via an opto-coupler, while synchronous rectifiers are self-driven from the transformer windings. There are two drawbacks in using this approach. First, the use of an opto-coupler generally limits the bandwidth of the regulation loop and the maximum ambient temperature and temperature of the printed circuit board (PCB) to less than about 85xc2x0 C. Secondly, the self-driven synchronous approach is generally not a good solution for higher frequencies.
In addition, protection such as over-voltage protection (OVP) has to be on the output side, which may require an additional opto-isolator just for over-voltage protection. Therefore, there is an advantage to having the control circuit on the output side. The major problem is to provide the necessary initial bias voltage before the converter is started. One possible solution is to have a separate isolated converter that will provide the bias voltage. Such a solution would require an additional magnetic core and, if realized employing planar magnetics, would consume a lot of board space.
The present invention provides a solution to the above problems. The apparatus of the invention employs a coreless isolated transformer, with associated electronic circuitry, for providing initial bias and enable signal for the control and drive circuitry referenced to the output of a converter. The improvement is accomplished by embedding the transformer primary and secondary windings into a multi-layer PCB so that the transformer does not occupy space on the top and bottom surfaces of the PCB The initial bias voltage is needed to initialize operation of the control circuit when referenced to the output side of the converter. Thus, complete regulation and drive signals are generated on the output side.
A coreless transformer does not use any magnetic core as do typical transformers. It is, therefore, important to provide, as best as possible, coupling between the primary and secondary windings with proper geometry and stack-up on the PCB. Magnetic coupling is through air so this structure will have small magnetizing inductance and large leakage inductance. The former imposes a limitation on volt-seconds that can be applied across the windings of the transformer, while the latter requires a proper turns ratio that would compensate for leakage inductance. In addition, by proper geometry (construction) of the windings of the coreless transformer, as well as stack-up of the PCB, leakage inductance can be minimized in order to achieve higher effective (actual) turns ratio.
Also, this transformer is optimized to operate at higher frequencies, for example, 500 kHz and above. Since there is no magnetic core, inductance of the winding of the coreless transformer is very small. Due to this fact, higher frequency operation is necessary to achieve reasonable usage, size and efficiency of the coreless transformer. It can be used in different ways:
a) To operate all the time, in which case it provides the necessary bias for the circuitry on the output side of the converter; or
b) To operate only for predetermined periods of time during start-up or re-start after fault conditions such as over-current or over-voltage protection, among others.
This mode is preferred, because of the low efficiency of the coreless transformer caused by relatively large magnetizing current.